The present invention relates to methods and compositions for treating subterranean formations. More particularly, the present invention relates to treatment fluids that comprise a nonnatural galactomannan and to methods of using such treatment fluids in subterranean treatments. The “nonnatural galactomannans” used in the treatment fluids of the present invention are galactomannans that have been isolated from at least part of a genetically modified plant.
Treatment fluids may be used in a variety of subterranean treatments, including, but not limited to, drilling operations, stimulation treatments, and sand control treatments. Drilling operations typically require the use of a drilling fluid. Drilling fluids typically require sufficient viscosity to suspend drill cuttings. One common production stimulation operation that employs a treatment fluid is hydraulic fracturing. Hydraulic fracturing operations generally involve pumping a treatment fluid (e.g., a fracturing fluid) that usually includes proppant particulates into a well bore that penetrates a subterranean formation at a sufficient hydraulic pressure to create or enhance one or more cracks or “fractures” in the subterranean formation. An example of a sand control operation is gravel packing. In gravel packing treatments, a treatment fluid carries particulates (commonly referred to as “gravel particulates”) to a desired area in a well bore, for example, near unconsolidated or weakly consolidated formation zones, to form a gravel pack neighboring that portion of the formation. In some situations, fracturing and gravel packing treatments are combined into a single treatment (commonly referred to as “frac-pack” operations).
Maintaining sufficient viscosity in these and other applications using treatment fluids is important for a number of reasons. One reason is to transport formation solids, such as drill cuttings, more readily. Another reason is to maintain desirable particulate transport characteristics. Additionally, maintaining sufficient viscosity may be important to control leak-off into the formation. To provide the desired viscosity, viscosifying polymers or “gelling agents” commonly are included in treatment fluids. Examples of commonly used polymeric gelling agents include, but are not limited to, guar gums, cellulose derivatives, and biopolymers.
Guar gum is an economical thickener and stabilizer extracted from the seed of the leguminous guar shrub, known as Cyamopsis tetragonoloba. Almost all the guar gum currently used in subterranean hydrocarbon recovery operations is imported from overseas. World events and fluctuations in crop yield can impact price structures for this vital component of the petroleum industry.
Guar gum is primarily composed of galactomannans. Galactomannans serve as storage carbohydrates that accumulate in the endosperm of seeds from which guar gum is derived. Galactomannans also accumulate in the endosperms of other leguminous plants, for example, locust bean (Ceratonia siliqua) and coconut palm (Cocus nucifera).
Galactomannans are formed from a linear (1→4)-β-linked mannan backbone with varying degrees of substitution with single-unit galactosyl side chains attached by (1→6)-α linkages. The production of galactomannans in plants is primarily dependent on the activity of two enzymes: mannan synthase, which makes β-1,4-linked mannan backbone, and α-galactosyltransferase, which adds galactosyl residues to the mannan backbone.